Stem cell factor (SCF) is a cytokine that mediates its diverse cellular responses by binding to and activating the receptor tyrosine kinase Kit (also known as SCF-receptor). Kit was initially discovered as an oncogene in a feline retrovirus that captured an activated and truncated form of the surface receptor (Besmer et al. (1986) J Virol 60: 194-203). SCF is encoded by the murine steel (SI) locus while Kit is encoded by the dominant white spotting (W) locus in the mouse (Copeland et al. (1990) Cell 63: 175-183; Huang et al. (1990) Cell 63: 225-233; Flanagan and Leder (1990) Cell 63: 185-194; Tan et al. (1990) Science 247: 209-212; Bernstein et al. (1990) Ciba Found Symp 148: 158-166; discussion 166-172). SCF functions as a non-covalent homodimer and both membrane-anchored and soluble forms of SCF generated by alternative RNA splicing and by proteolytic processing have been described (reviewed in Ashman (1999) Int J Biochem Cell Biol 31:1037-1051). Kit is a member of type-III family of receptor tyrosine kinases (RTK), which also includes PDGF-receptor-α, and β, CSF-1-receptor (also known as M-CSF-receptor or Fms), and the Flt3-receptor (also known as Flk2) (reviewed in Ullrich and Schlessinger (1990) Cell 61: 203-212; Blume-Jensen et al. (2001) Nature 411: 355-365). Kit is composed of a glycosylated extracellular ligand binding domain (ectodomain) that is connected to a cytoplasmic region by means of a single transmembrane (TM) domain (reviewed in Schlessinger (2000) Cell 103: 211-225). The ectodomain of Kit and other members of type-III RTKs all contain five Ig-like domains, in which the second and third membrane distal domains were shown to play a role in ligand recognition (reviewed in Ullrich and Schlessinger (1990) Cell 61: 203-212). Other RTKs whose extracellular ligand binding domains are composed exclusively of multiple Ig-like repeats include members of the VEGF-receptor family (7Ig-like), CCK4-receptor (7Ig-like) and FGF-receptors (3Ig-like). The cytoplasmic region of Kit contains a protein tyrosine kinase (PTK) domain with a large kinase-insert region; another hallmark of type-III RTKs. Binding of SCF to Kit leads to receptor dimerization, intermolecular autophosphorylation and PTK activation. It was proposed that the fourth Ig-like domain of Kit is responsible for Kit dimerization in response to either monovalent or bivalent SCF binding (Lev et al. (1992b) J Biol Chem 267: 15970-15977; Blechman et al. (1995) Cell 80: 103-113). However, other studies have demonstrated that ligand induced dimerization of Kit is driven by bivalent binding of SCF (Philo et al. (1996) J Biol Chem 271: 6895-6902; Lemmon et al. (1997) J Biol Chem 272: 6311-6317).
Characterization of mice mutated at the SCF or Kit loci has shown that SCF and Kit are required for development of hematopoietic cells, melanocytes, germ cells and intestinal pacemaker cells (reviewed in Ashman (1999) Int J Biochem Cell Biol 31:1037-1051). In humans, loss of function mutations in Kit cause the piebald trait that is characterized by de-pigmentation of the ventral chest and abdomen, white fareflock of hair, deafness and constipation (Fleischman et al. (1991) Proc Natl Acad Sci USA 88: 10885-10889). A variety of gain-of-function mutations in Kit were found in different types of human cancers. Activating Kit mutations were found in gastro-intestinal-stromal tumors (GIST), acute myeloid leukemia (AML) and mast cell leukemia (MCL) among other cancers. Mutations were identified in the membrane proximal Ig-like domain (D5) (exon 8 and 9), in the juxtamembrane (JM) domain (exon 11), and in the tyrosine kinase (PTK) domain (exon 17) (see Forbes et al. (2006) COSMIC 2005. BR J. CANCER, 94: 318-22. Somatic mutation database: Catalogue of Somatic Mutations in Cancer http://www.sangerac.uk/genetics/CGP/cosmic/). While there is good evidence that the gain of function mutations in the JM and the PTK domains lead to constitutive activation of Kit, by relieving autoinhibitory constraints (Mol et al., (2004) J Biol. Chem. 279: 31655-31663), the molecular mechanism underlying the gain of function mutations in D5 of the ectodomain is not understood. There is a need to better characterize the structures of RTKs such as Kit and PDGFR, as well as SCF, PDGFα/β, and the bound Kit/SCFcomplex. Such a characterization will lead to the informed identification of regions which may be targeted with drugs, pharmaceuticals, or other biologics.